I agree with you completely that all iguanas show that line when they turn etc. I can also say that most iguanas under 2 has that line even while laying down in a straight line. To test an iguana for dehydration it is best to do the pinch test on either the upper arm or leg. The urates are usually yellowish when they are severely dehydrated.

I do not believe that tough love works with an iguana, they are the most stubborn creatures I have ever worked with.

Although my iguana eats next to nothing during breeding season he still poops every single day. I am worried about your iguana, he is definitely doing things that raise concern and it can be more than just dehydration.

These care sheets may answer some of your questions.
Iguana Care Sheets & Information